The movement of finished printed articles from printing press to the shipping platform has long been one of the more difficult tasks in the publications industry. While the problem is experienced in the publications industry generally, it is most severe in newspaper production where the product, the finished newspaper, varies in size and weight from day to day, and, because of being folded along two adjacent sides and cut edges at its other sides, has greater thickness at the folded edges and has a top or bottom surface that is curved. As a consequence of their shape, it is difficult to form stacks of newspapers and to move those stacks at high speeds.
No better system has been devised than to receive newspapers from the press where they are printed, assembled, and folded, and then to stack a number of those newspapers flat, one atop the other, so that they can be tied, stored and distributed as bundled stacks of papers. In an effort to deliver newspapers whose news is as current as possible, the production of the newspaper is completed at the last possible time that permits the distribution system to perform its task. The presses can produce newspapers at rates in excess of twenty papers per second. The rate from the presses is so great that the steps of stacking and tying cannot be accomplished at the same work position. Instead, the tasks must be separated. The stack is formed at one position and is immediately conveyed away to another work station at which the stack is tied into a bundle. The task of tying the bundle, and the task of conveying the stack from the stacker to the bundler is greatly facilitated if half of the papers in a stack are rotated by 180.degree. relative to the remainder of the papers in the stack so that the folded edge of half of the papers overlies the cut edges of the other half of the papers. The need for such rotation increases as the thickness of the papers, and thus the height of the stack, is increased. The non-uniform shape that makes rotation necessary prevents rotation of half of a completed stack. Thus it is that the rotation must be accomplished during formation of the stack. In practice, papers are delivered to the stacking position, all oriented in the same fashion. Usually, they are delivered with the folded edge forward. When the papers have been stacked to half of the finished stack height, the half stack is rotated by 180.degree. and the upper half of the stack is placed on top of that rotated lower half.
Newspapers are produced and conveyed to the stacking position in a substantially continuous stream. However, at the stacker, the process becomes intermittant. The papers are placed on a platform which must remain stationary over the period in which the stack is formed, or over part of that period in the case in which part of the stack is rotated. As a consequence, stack rotation and discharge of completed stacks from the stacking position must be accomplished very rapidly. Because of their shape, the center of gravity of the stack of papers seldom falls on the rotational axis of the stacking. Further, the individual papers of a stack cannot be made to lie with their individual centers of gravity in a single line. As a consequence of that, rapid rotation of a stack gives rise to forces that tend to separate the papers and to disintegrate the stack. That is prevented by the use of retaining structures at all four sides of the stack to restrain it against disintegration. That restraining structure, at least at one side of the stack, must be removed before the stack can be discharged from the stacking position to make room for the next stack. The timing is such that discharge almost always must be accomplished positively by pushing the completed stack out of the stacking position.
The stacking table and the mechanism that rotates it, and the restraining structures, and the mechanism that removes those structures and the elements that push the papers out of the stacking zone, and the mechanism that operates that structure must all be designed to have sufficient strength to withstand the forces generated in accelerating and decelerating the newspaper stack, and, in their own acceleration and deceleration. Large forces are generated requiring the use of heavy drive mechanisms. That requirement further compounds the problem.
Thus it is that the transition from the more or less continuous process production of newspaper to intermittant processing at the stacker gives rise to some difficult problems. Stack rotation and discharge occur at different times, and, since the size of newspapers changes from day to day, it is not possible to link together the rotation and discharge mechanisms to produce a fixed, synchronous operation. Moreover, the flow of papers to the stacker frequently becomes non-uniform. Individual papers may become misaligned and become lodged to jam the operating mechanism unless that mechanism is arranged so that some variation in the time sequence of mechanism operation is possible. The problem that is faced by the designer of a stacker mechanism is how to divide a structure in which stacked rotation and discharge can be accomplished by separate mechanisms capable of rapid acceleration and deceleration to provide coordinated but non-synchronous function.